This invention relates to a heat-shrinkable polypropylene film adapted to be printed. More particularly, it relates to a heat-shririnkable film adapted to be printed for the purpose of, for example, obtaining a display effect for commercial packaging. The heat-shrinkable film of the present invention is particularly adapted to be printed with a flexographic ink having a poor adhesion to polypropylene resins.
Single layer heat-shrinkable films comprising a polypropylene resin or multiple films wherein at least one surface layer comprises a polypropylene film are in common use as heat-shrinkable packaging films. In addition, printed heat-shrinkable films have been used for increasing the display effect of packaged products. In printing these polypropylene heat-shrinkable films, a certain surface treatment is necessary before printing the films for improving adhesion between an ink and the film surface. As such surface treatment, corona discharge treatment has popularly been employed due to its ease. Such surface treatment has been inevitable in the case of printing by a flexographic ink.
However, conventional polypropylene heat-shrinkable films are wound so tight at ambient temperatures due to the shrinking properties of the films themselves (hereinafter referred to as spontaneous shrinking) that films stick to each other to cause blocking. This blocking tendency is particularly serious in the case of storing for a long time at elevated temperatures. In the printing process, the blocking causes change in film tension, resulting in shear in pitch of printed patterns, film breakage or, in an extreme case, film rupture.
In order to solve this blocking problem, it has been proposed to incorporate an amount of an anti-blocking agent composed of silica inorganic powder in the polypropylene heat-shrinkable films. This technique, however, has failed to provide favorable heat-shrinkable packaging films since a necessary amount of such anti-blocking agent for preventing the blocking in turn seriously decreases transparency or sealing properties of the film.
In another technique, a gravure ink having a somewhat better adhesion to polypropylene resins than the flexographic ink has been employed in order to print the film without previous corona discharge treatment. The thus printed films, however, have often suffered delamination of the printed ink when used under severe conditions. In addition, gravure printing process using the gravure ink requires an expensive printing plate, and hence it is not suited for small lot printing from an economic point of view.
Additionally, flexographic printing using the flexographic ink is suited for small lot printing since it requires only an inexpensive printing plate. As has been set forth hereinbefore, however, the flexographic ink has such a poor adhesion to the film that it fails to provide satisfactory printed films.